Re: [RFC/RFT PATCH] x86/efistub: Don't bother enabling SEV in the EFI stub

[Ard Biesheuvel]

Hi Tom,

Thanks for taking a look.


On Mon, 14 Apr 2025 at 21:10, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 4/14/25 08:04, Ard Biesheuvel wrote:
> > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >
> > One of the last things the EFI stub does before handing over to the core
> > kernel when booting as a SEV guest is enabling SEV, even though this is
> > mostly redundant: one of the first things the core kernel does is
> > calling sme_enable(), after setting up the early GDT and IDT but before
> > even setting up the kernel page tables.
> >
> > So let's just drop this call to sev_enable(), and rely on the core
> > kernel to initiaize SEV correctly.
>
> SEV support does some things in sev_enable() that aren't done later in
> the kernel proper. For example, we check certain hypervisor features and
> save the CC blob into the boot parameters.

I misread the last part: the kernel proper checks struct boot_params
and setup_data, whereas the decompressor checks both of those as well
as the EFI config table.

It would make sense for the EFI stub to populate the field in struct
boot_params directly.

> And as I look closer, I see
> that we don't call initialize_identity_maps() from the EFI stub so we
> aren't calling snp_check_features() from the EFI stub, which we should
> have been doing.
>

The EFI stub has its own check for this, based on the same underlying
logic (have_unsupported_snp_features()). But it checks this much
earlier so it can fail gracefully and return to the firmware, rather
than terminate.

> Just removing the call does cause an SNP boot to crash. My testing shows
> just ensuring that the cc_blob_address in the boot_params structure is
> set results in a successful boot under SNP. So some of sev_enable()
> needs to remain and some things need to be moved into the kernel proper.
>

Thanks, this is very helpful. I'll dig a bit deeper and try to address
the points you raised.